Composite structures may be fabricated using automated fiber placement machines to apply continuous strips or bands of tows of pre-impregnated fibers onto a tool or mandrel to form a composite layup having a plurality of plies. Each tow may be comprised of several tow segments that may be spliced end-to-end using resin. The tows may be drawn from a plurality of tow spools that may be mounted on a fiber placement head. The fiber placement head may include a tow collimator for aligning the plurality of tows into side-by-side arrangement with one another to form a tow band. From the tow collimator, the tow band may be fed to an application device of the fiber placement head where the tow may be pressed onto the tool or mandrel as the fiber placement head moves along the tool following the contour of the tool surface.
Occasionally, during the process of applying a tow band to a tool surface, a tow may be missing from the tow band such as due to a misfeed of a tow though the collimator or through other components of the fiber placement head. A tow may also be missing due to a break in a splice of the tow. In addition, a tow may be missing due to the tow falling off of the tool surface after the tow is applied to the tool due to insufficient tack between the tow and tool surface or substrate.
Conventional methods of detecting missing tows include visual inspection of the composite layup for the presence of depressions in the layup surface which may be indicative of a missing tow. Unfortunately, detection of a depression in the layup surface may be difficult due to the relatively small thickness (e.g., 0.008 inch) and relatively small width (e.g., 0.125 inch) of each tow. In addition, the difficulty in detecting a depression in the layup surface by visual inspection with the human eye may be compounded when the composite layup is comprised of overlapping plies having tows with the same fiber orientation resulting in minimal contrast due to the black color of the tow. Tactile inspection of the layup surface may be impractical for the relatively large amount of surface area associated with certain composite structures. In addition, tactile inspection may not yield the desired results.
A further drawback associated with conventional methods for detecting missing tows is that the fiber placement machine may continue to apply composite material with a missing tow in a tow band. For example, multiple plies of composite material may be laid over an area with a missing tow before the missing tow is detected and the fiber placement machine is stopped. In order to rework the area of the composite layup with the missing tow, the general area of the missing tow may be generally identified. However, rework of the composite layup may require the removal and replacement of material in a labor-intensive process and time-consuming process at significant cost. Furthermore, because the area of the missing tow(s) is only generally known, large amounts of material may require removal before reaching the exact location of the missing tow.
As can be seen, there exists a need in the art for a system and method for detecting a missing tow in a composite layup with improved accuracy. In addition, there exists a need in the art for a system and method for detecting a missing tow in a composite layup wherein the exact location of the missing tow may be identified prior to minimize the amount of composite material that is removed during the rework process.